1. Field of the Invention
The present invention relates to a vibrating mirror (or a deflecting mirror) used in, for example, optical scanners, optical-scan display devices, or in-vehicle laser radars. The present invention also relates to an optical scanner with a vibrating mirror, and to an image reproducing/forming apparatus, such as digital copying machines, laser printers, laser plotters, laser facsimile machines, etc., employing the optical scanner.
2. Description of Related Art
In conventional optical scanners, polygon mirrors or galvanometer mirrors are used to deflect beams for writing images. In order to achieve high-resolution high-speed printing operations, the rotational speed of these mirrors has to be increased. However, there is a ceiling to increasing the rotational speed of the mirror because of various reasons, such as limitation in durability of the bearings, heat generation due to windage, and noise.
On the other hand, optical deflectors making use of micromachining of silicon have been researched and studied. For example, JP 4-211218A and JP 11-52278A, which issued as Japanese Patent Nos. 2924200 and 3011144, respectively, disclose a technique for monolithically and integrally fabricating a vibrating mirror, together with a torsion bar supporting the mirror on its axis, from a silicon substrate. One of the advantages of the integrally fabricated vibrating mirror with the torsion bar is that the reciprocating motion of the mirror is produced by resonance, and that high-speed operation is achieved. In addition, noise and power consumption are reduced because less driving force is required to swing the vibrating mirror.
However, this type of vibrating mirror is incapable of deflecting a light beam over a wide range, unlike the conventional polygon mirror, because the size of the mirror surface and the sweep angle are small. To overcome this problem, JP 2002-258183A proposes to arrange multiple optical scanning units, each using a vibrating mirror as a deflector, such that the scanning directions of the optical scanning units align with each other in the fast scan direction. Under this structure, the entire imaging range (or the writing range) is divided into several sections along the scanning line.
In general, as the mirror surface becomes large, the mass increases and the sweep angle decreases. This is because the force of inertia acts on the end portions of the mirror opposite to the rotational force acting on the torsion bar. The viscosity resistance of the air acting on the mirror surface also narrows the sweep angle.
JP 2001-249300A proposes to arrange hollow areas or recesses on the rear side of the mirror substrate to reduce the mass. JP 5-153338A proposes to place the vibrating mirror in a vacuum vessel and seal up the vessel in order to reduce the viscosity resistance and the driving voltage. On the other hand, JP 2003-15064A and JP 2003-503754A propose to couple the torsion bar to the mirror substrate at several positions for the purpose of preventing the mirror from vibrating in directions other than the direction of rotation.
The technique of dividing the entire imaging region into several sections in the fast scan direction is advantageous because each of the optical scanning units can be made compact, reducing the scanning width and the optical path length. Accordingly, a low-noise and power-saving image reproducing/forming apparatus is realized, using micromirrors capable of low-load optical scan. However, when the dimensions of the mirror surface are increased, the rotational force for driving the mirror has to be increased to guarantee the sweep angle against the increased force of inertia.
The mirror substrate is as thin as 100 μm. The force of inertia acting on the mirror increases as the working point separates from the rotational axis (or approaches the mirror end), and shearing stress is generated in the mirror substrate against the rotational force propagating from the torsion bar. As a result, the mirror surface bends in a sinusoidal curve and the surface accuracy is degraded.
The force of inertia acting on the mirror substrate increases as the mirror angle (sweep angle) approaches the maximum because the negative acceleration applied on the mirror substrate increases. The wider the sweep angle, the less the surface accuracy is.
To guarantee surface accuracy, the effective scan ratio, that is, the ratio of the actual sweep angle employed in image formation to the maximum sweep angle, has to be reduced. This is one of the factors that makes it difficult to increase the sweep angle to extend the imaging range even under an increased rotational force applied to the mirror. The number of divided sections along the scanning line may be increased to compensate for the limited sweep angle; however, this results in undesirable increased cost.
FIG. 1 is a schematic diagram illustrating the effect arising when the light flux is incident on the curved surface of the mirror. In reality, the quantity of the curvature on the mirror surface is at the wavelength level, and is sufficiently small with respect to the tilt of the mirror surface. The drawing is rather exaggerated.
It is assumed that a single light flux 353 with a rectangular cross-section is incident on the mirror surface covering the crest 351 and the trough 352. In this illumination area, the radius of curvature of the mirror surface varies in the fast scan direction. The focusing point of the light flux component 355 having been reflected from the crest 351 goes away from the focusing point of the light flux component 354 reflected from the center of the mirror, due to the convex mirror effect. To the contrary, the focusing point of the light flux component 356 reflected from the trough 352 comes closer due to the concave lens effect. Since the light flux is divided into different portions when reflected from an uneven surface, the profile (intensity distribution) 357 of the beam spot on the scanned plane has a side lobe with wide skirt. This phenomenon causes the image to blur, reduces the resolution, and degrades the image quality.